1. Field of the Invention
The invention relates to a torque stabilizing agent, to a torque stabilizing method for fasteners, and to torque stabilized fasteners. More particularly, the invention relates to a torque stabilizing agent which comprises an organic film forming material and about 2% to about 25% by weight of a surface active agent, to a torque stabilizing method for fasteners in which said torque stabilizing agent is employed, and to fasteners whose torque has been stabilized by said torque stabilizing method.
A fixed relation between the fastening torque and the axial force (the fastening force) is desirable when bodies are fastened together with fasteners, such as bolts, nuts and the like, in order to prevent the defect of said fastening parts. The relation between the fastening torque and the axial force is generally represented by the following formula ##EQU1## wherein T is the fastening torque, Q is the axial force, .mu. is the coefficient of friction between the fastener and the body it is fastened to, P is the pitch of the thread, de is the pitch diameter of the thread and dN is the friction diameter.
Referring to formula (1), it is clear that the relation between the fastening torque and the axial force depends on the coefficient of surface friction of the fastener. Therefore, it is essential to stabilize or fix the coefficient of surface friction in order to fix the relation between the fastening torque and the axial force. However, the coefficient of surface friction of the fastener is remarkably dependent on the surface condition of the fastener. For example, treatment of the fastener surfaces, such as by zinc plating, by phosphate treatment, by zinc chromate treatment and the like, treatment of the fastener surfaces with an adherant, such as machine oil, cleaning materials and the like, and corrosion on the fastener surfaces determine the surface condition and, hence, effect the coefficient of surface friction of the fastener.
The relation between the fastening torque and the axial force changes according to the change of the coefficient of surface friction as is shown by formula (1), and so it is difficult to stabilize the torque of the fastener without regard to the surface conditions. Therefore, the defect of the fastening part, such by disconnection, damage and the like, may not be avoidable when the surface conditions of the fastener varies.
It should be understood that a torque stabilizing agent prevents disconnection of or damage to fasteners by maintaining a fixed relationship between the fastening torque and the axial force regardless of the surface conditions. This is accomplished by maintaining a fixed coefficient of friction between the surfaces in contact with each other regardless of their conditions.
It should also be understood that a useful torque stabilizing agent should provide a coefficient of surface friction which is neither too high nor too low. If the coefficient of friction is too low, the fastening parts will become too easily disconnected. To prevent disconnection, a workman may apply excessive torque, which can result in damage to or rupture of the fastening parts. On the other hand, if the coefficient of friction is too high, fastening the parts together will be difficult. Again, a workman is likely to apply too much torque, which can result in damage to or rupture of the fastening parts. Furthermore, a composition which cures after application will prevent disconnection of the fastening parts even when such disconnection is desired.
That the coefficient of friction should be neither too high nor too low may also be demonstrated by referring to formula (1). It may be seen that Q (the axial force) is a linear function of T (the fastening torque), the slope of which includes 1/.mu. (the coefficient of surface friction). Q changes very rapidly with T when the value of .mu. is low, but very slowly when the value of .mu. is high. Therefore, for low values of .mu., it is very difficult to control Q. A small increase of T will result in rupture of the fastener, while a small decrease of T will result in disconnection of the fastener. On the other hand, for high values of .mu., it is very difficult to achieve a sufficiently high value of Q by increasing T. Thus, the rupture point of the fastener will be reached over a wide range of values for T while Q remains low.
Thus, an optimum range exists for the coefficient of surface friction in order to achieve a preferable fastening state. The optimum range for the coefficient of surface friction is generally about 0.1 to 0.2.
The torque stabilizing agent of this invention is employed to stabilize or fix the coefficient of surface friction of the fastener within the optimum range regardless of the surface conditions. It thus prevents the defect of the fastening part.
2. Description of the Prior Art
Hitherto, a few kinds of torque stabilizing agents have been proposed to stabilize the relation between the fastening torque and the axial force. One of them is a kind of lubricant such as silicone oil, paraffin oil, wax and the like. Said lubricant is not satisfactory. It tends to degenerate in the atmosphere. The bond between said lubricant and the surface of the fastener is very low so said lubricant is easy to peel from the surface of the fastener. Furthermore, it provides a coefficient of friction which is too low. A kind of synthetic resin paste has also been proposed as a torque stabilizing agent. Said synthetic resin paste also is not satisfactory. It has high viscosity and the thread of the fastener is filled up by said synthetic resin paste. It is very difficult to screw in a fastener when the thread is filled up with the synthetic paste. Moreover, it is necessary to prevent said synthetic resin paste from adhering to the head of the fastener because it is difficult to fit the clamping tool, such as spanner, wrench and the like, onto a fastener head to which said synthetic resin paste adheres.